Imaging devices of both planar, external drum and internal drum design, such as photoplotters and scanners, are known in the art. Scanners function by illuminating the test sample with an unmodulated optical beam and capturing the reflected or transmitted light after it leaves the copy. The transmitted or reflected optical signals are received by a detector and recorded.
Photoplotters are used in the field of graphic arts and the fabrication of printed circuit boards. Photoplotters expose a photo-sensitive media plate disposed on an imaging surface by sweeping a modulated optical beam over the media plate. The modulated optical beam is provided by a beam generator.
A planar photoplotter such as are disclosed and claimed in U.S. Pat. No. 4,851,656 and incorporated herein by reference are types of imaging systems which have a planar surface for receiving a substrate or media. An optical exposure head is located on a movable gantry apparatus and is rastered above the media during exposure.
Internal drum devices have a cylindrical surface portion to receive the media. An optical beam generator emits a modulated optical feed beam onto a spinning mirror of a scanning assembly, and the mirror reflects the beam onto the media. As the mirror spins, the reflected imaging beam advances across the media surface from one side edge of the surface to an other side edge thereof, exposing a sequence of pixels which together form a scan line perpendicular to the axis of the drum. The spinning mirror is mounted on a carriage which moves along the axis of the drum, perpendicular to the scan line. The carriage moves continuously so that the imaging process is helical along the cylinder. The rotating imaging beam advances across the surface area of the drum in this manner until the entire image is exposed to the media.
The model Crescent 42 internal drum photoplotter presently manufactured by the assignee of the present invention, Gerber Scientific, Inc., includes a carriage having two generally orthogonal surfaces, which constrain the carriage in two planes, magnetically coupled to a rigid spar that extends along the longitudinal axis of the drum. The carriage is suspended below the spar. A plurality of friction pads formed of polymeric material are secured to the orthogonal surfaces of the carriage for maintaining the carriage a predetermined distance from the spar. A spinner motor and mirror for reflecting the optical beam to the media is mounted to the under surface of the carriage.
The carriage is driven along the spar by a drive system comprising a lead screw and a stepper motor. A motor drive system energizes the stepper motor in minute increments to rotate the lead screw resulting in movement of the carriage along the spar. The motor drive system controls the stepper motor in an open loop mode.
The quality of the imaging with the Crescent 42 photoplotter is limited by the smoothness of the movement of the carriage along the spar. Any variation of the rate of the movement of the carriage results in misplacement of the image and, in the worse case, "banding" or longitudinal lines formed in the media. The combination of the friction between the spar and friction pads, the tolerances in the grinding and lapping of the lead screw, the incremental indexing of the stepper motor, and the lack of feedback of the carriage position all contribute to the variation in the velocity of the carriage.
As detailed hereinabove, the lead screw must be lapped to extremely tight tolerances. Very few manufactures are able to manufacture an acceptable lead screw which is of a length of approximately 42 inches. Typically, a lead screw must be further lapped by hand once received from the manufacturer for assembly in the photoplotter. The tight tolerances and additional hand lapping increase the manufacturing costs of the lead screw.
Another concern in the industry is the cycle time for loading, scanning and unloading each plate of media. A decrease in the time to handle and scan the media plate will increase production of the plates resulting in a reduction in equipment cost, floor space, response time (time to image the first in a series of plates), and manpower requirements. It is particularly valuable to increase the throughput rate of the larger imaging system (68 inch format) which has the longest imaging time and the highest equipment cost. The 68 inch format imaging system is capable of imaging sixteen individual pages of 8.5.times.11 inch format arranged in a 4 by 4 page layout on a single plate of media.
One limitation to the cycle time of the imaging system having a lead screw drive system for the carriage is that the maximum rate of movement of the carriage along the spar is relatively slow. This slow rate is evidenced by the relatively slow scram or slew rate for returning the carriage back to its initial position which increases the time to unload the scanned media plate after it is scanned.
The majority of the cycle time for a plate of media is the amount of time it takes to scan or image the media, especially for the larger formats. The scan rate of an internal drum imaging system is limited by the rotation rate of the scanning monogon mirror. A typical monogon scan rate is 18,000 rev/min or 300 scans/sec. The time to image a plate is equal to the slow scan travel along the length of the plate multiplied by the imaging resolution divided by the scan rate. This calculation yields an imaging time of 203 sec (3.4 min.), 356 sec (5.9 min.), 576 sec (9.6 min.) for plate lengths of 24, 42, and 68 inches, respectively, at a typical imaging resolution of 2540 scans/inch. If a plate handling time of 60 seconds per load/unload cycle is added to these times; the plate imaging rates would be 13.7, 8.7, and 5.7 plates per hour for plate lengths of 24, 42 and 68 inches, respectively.
Accordingly, it is the general object of the present invention to provide a scanning system for an imaging device that increases the production rate of plates of media.
It is another object to provide a scanning system for an imaging system having multiple independent scanning assemblies for increasing the rate of scanning of the plate of media.
It is a further object to provide a scanning system that has a greater scram or slew rate for reducing the cycle time to unload the scanned plate of media.
It is yet another object to provide a scanning system capable of simultaneously scanning a plurality of images having different imaging parameters on the same plate of media.
It is still another object to provide a scanning system that provides redundancy to enable an imaging device to scan the media if a scanning assembly malfunctions.
It is a further object to provide a scanning system capable of selecting a plurality of types of beam generators to scan the media.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.